Here is our setup

  • Outlet in older house that was replaced with a GFI last year. (no issues until recent)

  • The GFI doesn't have a ground wire running up from the basement. Just the white and black wires. (From my understanding this is an acceptable configuration)

  • The outlet reads 119.0v when checked with my multi-meter.

  • The outlet reads less than a volt when the "positive" line and the ground are checked with the multimeter. Same with the "negative" with the ground.

  • A (real) surge protector plugged into the outlet

  • A large tv plugged into the surge protector

  • A cox cable box plugged into the surge protector

  • A coax cable (run by cox) from the outside of the house, split and sent to a modem and the cable box. The coax reads 0v (no leakage at all) when a multimeter is placed on the inner coax wire and the outer shielding. I tried this on both DC and AC settings on multimetter

The problem:

When either a coax cable or HDMI cable is run from the cable box to the TV, an Arc appears. This has already fried one TV.

My theory:

The coax cable is improperly grounded. I just need a way to prove it. The basis for this theory is the coax cable in a separate apartment tests with an AC voltage of 1.5v. The cable technicians are somehow less informed about circuitry than me. This is a hobby for me so I am lost in the sauce. Any Ideas?


So I took the time to diagram what I have observed. This further supports my theory that the cable isn't grounded properly. I'm not 100% sure how coax is supposed to be run from a telephone pole though so I could be incorrect.

Just in case there is confusion, no the neutral is not plugged into the ground. I was just demonstrating that there is only two wires going to the GFI.

enter image description here


5 Answers 5


Near where the service enters the house, you should see a grounding block similar to this.

Coaxial cable grounding block

The coaxial cable from the pole/dish/mast should be connected to one side, and the coaxial cable feeding to the house should be on the other side. The screw terminal should have an insulated copper wire, larger than 14 AWG, and shorter than 20' (6 m) attached. The other end of this wire should be attached to either:

  • The Intersystem Bonding Termination.
  • The grounding electrode system.
  • The grounded interior metal water piping system, within 5' (1.5 m) from the point of entrance to the building.
  • The metallic power service raceway.
  • The service equipment enclosure.
  • The grounding electrode conductor.
  • Or where the building has no grounding means, to an approved grounding electrode.

If the service is not grounded at the house, it could be (and likely is) at a different voltage potential than the grounding system in your house.

  • I tested the outlet today. Its wiring is fine. The only remaining possibilities is either that both cable boxes brought in were faulty or that the grounding is screwed up within the cox external box. Commented Sep 16, 2014 at 0:40

Something is certainly wired wrong or malfunctioning badly. It could be the cable box. It is less likely to be the cable, but not impossible.

Isolate the problem. Disconnect the cable from the cable box, and carefully use a voltmeter to check the voltage between the cable box's output and the TV's input. (The coax connection would be easier to check; measure both ground-to-ground and signal-out-to-signal-in.) If that gives you a high reading, the cable box, surge protector, or house wiring are at fault. In that case, try plugging the cable box directly into the wall and test again to eliminate the surge protector. If the problem doesn't go away when plugged in directly, I'd call the cable company; the problem is either their box, their line, or your house wiring (needing an electrician to investigate properly).

If you didn't get a bad reading from any of the tests in the first paragraph, check the voltage between the cable box and the cable. It should be quite low. If it's anything near line current, then call the cable company and and get them to investigate; the problem may be their equipment, or possibly another customer (I don't know how well they isolate customers from each other).

Actually, I'd probably start by calling the cable company. Tell them what you've told us, tell them that you think it's their equipment at fault, and make it their job to prove otherwise. Among other things, if it IS their equipment and wasn't just a lightning strike, they may (or may not) owe you something for that killed TV.

(One of the advantages of fiber optics is that there's no way it can put an electrical surge into your equipment...)

  • So far it has destroyed a tv and two cable boxes. This leads me to believe that it's at least not the cable box itself. Also this entire setup worked fine with a small 20" tv but seems to be present with the larger TVs 40"+ Commented Sep 14, 2014 at 22:07
  • OK, I'd still try making it the cable company's problem... since otherwise I'm inclined to suspect your house wiring is Badly Wrong.
    – keshlam
    Commented Sep 14, 2014 at 22:08
  • Is it possible that the house wiring is wrong even though the (1 year old GFI) outlet is producing 119v when tested? Commented Sep 14, 2014 at 22:09
  • Unless everything is plugged into the same outlet via a single power strip, it's possible that one of the two-or-more outlets is being fed through a different circuit. Normally, that shouldn't be a problem (beyond possibly introducing "ground loop" interference), but if one of those feeds is malfunctioning and somehow putting voltage onto safety ground, all bets are off. So, yeah, plugging everything into a single outlet via power strips might be another thing to check. Or jump straight to checking voltage across the cable input and/or between the cable and your equipment.
    – keshlam
    Commented Sep 14, 2014 at 22:16
  • 1
    It's possible that the neutral and hot of the outlet are swapped. One way to check is to use a non-contact voltage detector to make sure that voltage is on the hot terminal of the outlet but not the neutral. This check isn't 100% accurate, but it generally works. Also, check the voltage (AC and DC) between the shield of the COAX and the outlet's neutral terminal. They should be close to zero.
    – Pigrew
    Commented Sep 15, 2014 at 3:21

To check that the coax is grounded properly you need to measure between the outside of the coax cable and ground - preferably the ground pin of an outlet that actually has a ground. It should read 0v or close to it. If there are other properly grounded outlets you might be able to run an extension cable and use its ground to test. You should also measure from an actual ground pin to something metal on your cable box - it should also read near 0V.

You said they added a ground to the water spout outside - but did they check that the spout is actually grounded? If your house has any pex plumbing it may not be. I suspect this is the problem.

Ground isn't actually 0v - if you could measure from my front yard to the other side of the city there could be hundreds of volts difference. This is why all cables (power, phone, cable, etc) need to be grounded at or near the same place to eliminate these differences.

What is likely happening is that the cable isn't really grounded at your house - but it is grounded at the other end somewhere, so there is a voltage difference between your house's ground level and the ground of the coax. This can be enough to give you a shock or destroy electronics.

The first thing I would do is check if that water spout is grounded. If it is not, find a spot that is and insist that the cable company come out and run the ground to a proper spot.


( Do not directly touch anything conductive while doing the following test. )

As a quick check, test if there is voltage between the shield of the coax and the outlet.
There should be no DC volts anywhere.
The voltage between the coax and the neutral should be very near 0V AC.
The voltage between the coax and the hot should be near 120V AC.

If both checks come up with near 0V AC then your coax isn't grounded. If this is the case ground it and test again. A piece of wire on the other end of the coax stuck in the dirt should be good enough for testing. You will just get lower readings than you should.

If both checks come up with at least a few Volts AC your neutral may not be properly grounded at the main disconnect. Another possibility is the coax is ungrounded or "grounded" to something other than a proper ground.

If the voltages are swapped either the wires to the outlet are swapped somewhere, or the coax may be connected to the hot somewhere.

If you get significantly more than 120 the coax is probably connected to the hot somewhere.

I'm going to have you also check that the neutral is properly connected to ground by checking the voltage between the neutral and ground pins of an outlet that is actually grounded. ( The ground and neutral wires should be connected in exactly one place, very near where the power comes in. ) It should be very near 0V if it's wired correctly. It is unlikely to be exactly 0V AC unless there is absolutely no load on this circuit.

You can also check if the ground has a good enough connection to the earth by testing the voltage between the ground pin of a grounded outlet and the dirt a few feet away from the house. It should definitely be less than 5V AC. ( It will only be at 0V AC if the load on the entire system is balanced. Which is very unlikely )
If it is more than 5V AC either you have a grounding problem, or your neighborhood has a grounding problem, or you have a very unbalanced load. ( 5V is just an arbitrary cut off point )

If either of the last two checks fails call an electrician, and tell them that you appear to have a problem with your grounding system.

If after an electrician has checked your grounding system you still have more than 5V AC between ground wire and the earth call your electrical utility and tell them that they either have a grounding problem, or a very unbalanced load.

Before calling it done, touch the outside of the coax to a known good ground. The cover plate screw of a properly grounded outlet or switch should work for this. If you get a spark you're not done yet, and I doubt that I can help you much more from here.


This is a classic sign of a hot/neutral reverse or current being induced or shorted in to the coax.

First step is to test your outlet and power strip. If you don't have an outlet tester that can indicate a hot/neutral reversal, use a multimeter to measure the voltage between the left prong slot on the outlet and the ground (assuming the outlet is oriented so the ground is on the bottom). This should read 0 volts. Anything else indicates a wiring problem here. Fix this first. Hire an electrician.

To test for current on the coax, use a multimeter to measure the voltage between the center conductor and the shield, the center conductor and the outlet neutral and ground, and the shield and the outlet neutral and ground. If any these measure a voltage, then disconnect the coax from your provider and test that cable sane ad you did inside. Also test where your house wiring that the providers cable is attached to. That will give you a good idea of where the current is entering the system.

  • I tested the outlet today with a standard outlet tester from radio shack. It says that the outlet is wired correctly. We called the cable company to come and check the coax and thr grounding within the box. They will be here friday, Commented Sep 16, 2014 at 0:39
  • Another possibility is that another cable device like a cake box in another room or a cable modem or an amplifier is connected to an outlet that is reversed. Check all of your other cable equipment with your outlet tester.
    – longneck
    Commented Sep 16, 2014 at 3:28
  • The standard outlet testers cannot tell if the neutral and ground are both hot, and the hot is neutral, perhaps especially in this case since there is no safety ground.
    – Pigrew
    Commented Sep 17, 2014 at 5:37
  • @Pigrew good point.
    – longneck
    Commented Sep 17, 2014 at 10:44

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